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DNA The Genetic Material. Scientific History. The march to understanding that DNA is the genetic material T.H. Morgan (1908) Frederick Griffith (1928) Avery, McCarty & MacLeod (1944) Erwin Chargaff (1947) Hershey & Chase (1952) Watson & Crick (1953) Meselson & Stahl (1958). - PowerPoint PPT Presentation
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DNAThe Genetic Material
Scientific History The march to understanding that DNA is
the genetic material T.H. Morgan (1908) Frederick Griffith (1928) Avery, McCarty & MacLeod (1944) Erwin Chargaff (1947) Hershey & Chase (1952) Watson & Crick (1953) Meselson & Stahl (1958)
Chromosomes related to phenotype T.H. Morgan
working with Drosophila fruit flies
associated phenotype with specific chromosome white-eyed male had specific
X chromosome
1908 | 1933
Genes are on chromosomes Morgan’s conclusions
genes are on chromosomes but is it the protein or the
DNA of the chromosomes that are the genes? initially proteins were thought
to be genetic material… Why?
1908 | 1933
What’s so impressiveabout proteins?!
The “Transforming Principle” 1928
Frederick Griffith Streptococcus pneumonia bacteria
was working to find cure for pneumonia
harmless live bacteria (“rough”) mixed with heat-killed pathogenic bacteria (“smooth”) causes fatal disease in mice
a substance passed from dead bacteria to live bacteria to change their phenotype “Transforming Principle”
The “Transforming Principle”
Transformation = change in phenotypesomething in heat-killed bacteria could still transmit disease-causing properties
live pathogenicstrain of bacteria
live non-pathogenicstrain of bacteria
mice die mice live
heat-killed pathogenic bacteria
mix heat-killed pathogenic & non-pathogenicbacteria
mice live mice die
A. B. C. D.
DNA is the “Transforming Principle” Avery, McCarty & MacLeod
purified both DNA & proteins separately from Streptococcus pneumonia bacteria which will transform non-pathogenic bacteria?
injected protein into bacteria no effect
injected DNA into bacteria transformed harmless bacteria into
virulent bacteria
1944
What’s theconclusion?
mice die
Oswald Avery Maclyn McCarty Colin MacLeod
Avery, McCarty & MacLeod Conclusion
first experimental evidence that DNA was the genetic material
1944 | ??!!
Confirmation of DNA Hershey & Chase
classic “blender” experiment worked with bacteriophage
viruses that infect bacteria grew phage viruses in 2 media,
radioactively labeled with either 35S in their proteins 32P in their DNA
infected bacteria with labeled phages
1952 | 1969Hershey
Why useSulfur
vs.Phosphorus?
Protein coat labeledwith 35S
DNA labeled with 32P
bacteriophages infectbacterial cells
T2 bacteriophagesare labeled with
radioactive isotopesS vs. P
bacterial cells are agitatedto remove viral protein coats
35S radioactivityfound in the medium
32P radioactivity foundin the bacterial cells
Which radioactive marker is found inside the cell?
Which molecule carries viral genetic info?
Hershey & Chase
Blender experiment Radioactive phage & bacteria in blender
35S phage radioactive proteins stayed in supernatant therefore viral protein did NOT enter bacteria
32P phage radioactive DNA stayed in pellet therefore viral DNA did enter bacteria
Confirmed DNA is “transforming factor”
Taaa-Daaa!
Hershey & Chase
Alfred HersheyMartha Chase
1952 | 1969Hershey
Chargaff DNA composition: “Chargaff’s rules”
varies from species to species all 4 bases not in equal quantity bases present in characteristic ratio
humans:
A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
1947
That’s interesting!What do you notice?
RulesA = TC = G
Nucleic Acids
Building blocks are nucleotides
Each nucleotide made up of A 5-carbon sugar
RNA: ribose DNA: deoxyribose
Nitrogenous base Adenine, Cytosine,
Guanine, Thymine (DNA only), Uracil (RNA only)
Phosphate group(s)
Basic Structure of a Nucleotide
Structure of DNA Watson & Crick
developed double helix model of DNA other leading scientists working on question:
Rosalind FranklinMaurice WilkinsLinus Pauling
1953 | 1962
Franklin Wilkins Pauling
Watson and Crick1953 article in Nature
CrickWatson
Rosalind Franklin (1920-1958)
But how is DNA copied? Replication of DNA
base pairing suggests that it will allow each side to serve as a template for a new strand
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” — Watson & Crick
Models of DNA Replication Alternative models
become experimental predictions
conservative semiconservative
Can you designa nifty experiment
to verify?
dispersive
1
2
P
Semiconservative replication Meselson & Stahl
label “parent” nucleotides in DNA strands with heavy nitrogen = 15N
label new nucleotides with lighter isotope = 14N
“The Most Beautiful Experiment in Biology”
1958
parent replicationMake predictions…
15N parent strands
15N/15N
Predictions
1st round of replication
conservative
15N/15N
14N/14N
semi-conservative
15N/14N
dispersive
15N/14N
conservative
15N/15N
14N/14N
semi-conservative
15N/14N
dispersive
15N/14N
2nd round of replication
14N/14N
15N parent strands
15N/15N
1
2
P
Franklin Stahl
Matthew Meselson
Matthew Meselson Franklin Stahl
Meselson & Stahl
Scientific History March to understanding that DNA is the genetic material
T.H. Morgan (1908) genes are on chromosomes
Frederick Griffith (1928) a transforming factor can change phenotype
Avery, McCarty & MacLeod (1944) transforming factor is DNA
Erwin Chargaff (1947) Chargaff rules: A = T, C = G
Hershey & Chase (1952) confirmation that DNA is genetic material
Watson & Crick (1953) determined double helix structure of DNA
Meselson & Stahl (1958) semi-conservative replication
proteinRNA
The “Central Dogma”
DNAtranscription translation
replication
Flow of genetic information in a cell
Science …. Fun
Party Time!
Any Questions??
Ghosts of Lectures Past(storage)
Semiconservative replication Meselson & Stahl
label “parent” nucleotides in DNA strands with heavy nitrogen = 15N
label new nucleotides with lighter isotope = 14N
“The Most Beautiful Experiment in Biology”
1958
parent replication
15N parent strands
15N/15N
Semiconservative replication Make predictions…
15N strands replicated in 14N medium 1st round of replication? 2nd round?
1958
where should the bands be?
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